This invention relates generally to a voltage-controlled oscillator (VCO) for use with a radio transmitter or receiver of frequency synthesizing type, and more particularly, the present invention relates to VCOs using a plurality of voltage-variable capacitance or varactor diodes.
Some conventional VCOs employ one or two varactor diodes so that oscillating frequency can be changed in accordance with a D.C. bias or control voltage applied to the varactor diodes. However, such conventional VCOs have suffered from a drawback if the carrier-to-noise (C/N) ratio deteriorates when the VCO is used for a wide frequency band, such as over 5 percent in terms of band ratio, or when a low D.C. voltage is applied to the varactor diode. In detail, when it is intended to cover a wide frequency range by a single tuning circuit, the capacitance of the varactor is selected to be a relatively small value compared with a coupling capacitor which is connected in series with the varactor. Accordingly, the voltage of the high-frequency signal applied to the varactor is relatively large, resulting in deterioration in C/N. Furthermore, when it is intended to cover a wide frequency band, the D.C. bias voltage applied to the varactor may be selected to be a relatively low voltage so that capacitance thereof changes widely in response to a small change in the D.C. bias voltage. However, nonlinear distortion of a varactor per se is large at such an operating point of a low D.C. bias voltage. Therefore, distortion in capacitance variation due to application of a high-frequency voltage is remarkable. In addition to the above reason, the value of Q of a varactor is apt to lower when such a low D.C. bias voltage is given. For these reasons, when a varactor operates at a nonlinear region in accordance with an applied high-frequency voltage, the C/N is apt to deteriorate.
In conventional wideband radio equipment which is required to show a carrier-to-noise ratio of over 90 dB as in a radio receiver, therefore, two or more VCOs are employed in such a manner that each VCO covers a given frequency range so that a desired wideband is covered by the plurality of frequency ranges of the VCOs.